Direct conversion receivers are desirable in part because they convert signals of interest directly to baseband (or near zero hertz) from a radio frequency (RF) or an intermediate frequency (IF). Simple direct conversion receivers, such as receiver 10 shown in FIG. 1, suffer from multiple drawbacks. The RF signal f.sub.1 is mixed with the local oscillator signal f.sub.0, and the signal of interest f.sub.1 -f.sub.0 is produced at baseband at the output. Unfortunately, superimposed on the signal of interest is the image f.sub.0 -f.sub.1. The "image problem" of simple direct conversion receivers is well known in the art of receiver design, the solution to which has been the subject of scholarly study for decades.
Image reject mixers, such as mixer 20 in FIG. 2, have been developed in response to the image problem suffered by simple direct conversion receivers. The operation of image reject mixers, including the mathematical basis upon which they operate, is described in detail in "High-Performance, Single-Signal Direct-Conversion Receivers" by Rick Campbell, published in the January, 1993 issue of QST magazine. Image reject mixers utilize two local oscillator signals, each differing from the other by 90 degrees in phase. Image reject mixers also require the use of two separate mixer elements. Image reject receivers represent a complex and expensive solution to the image problem of direct conversion receivers.
Both simple direct conversion receivers and image reject mixers nominally exhibit a loss of 6 dB because half of the signal is converted to f.sub.0 +f.sub.1, the sum of the RF frequency and the local oscillator frequency, and then discarded. In practice, the loss is often greater than 6 dB because conventional mixers are typically implemented with diodes which exhibits a finite amount of loss themselves. Typical conversion loss in prior art image reject mixers is 7-8 dB.
What is needed is a low-loss method and apparatus for simply and inexpensively overcoming the image problem of direct conversion receivers.